I think more people look at BB because it's simpler. If the big blind is 1200 then you multiply by 10 (just add a zero), and you know where you stand. If you have more than 12,000 then you're above 10x and if you have less, then you are below 10x. The point is that it is a simple calculation whether you choose divide your stack or multiply the blinds. To calculate your M value you have to multiply the ante by the number of players at the table, then add the values of the BB and SB to that. Once you have that number, you can divide your stack by it to find the M value. Although it's not terribly complicated, it's still easier to determine how many BB you have by performing one quick calculation. Not to mention, as you near then end of a tourney (when M value becomes most important), players are going to start being eliminated with greater frequency. Which means you will have recalculate you M value every time a player is eliminated or tables are rebalanced. So for the sake of simplicity most players will just calculate BB.
As for shoving or folding with 10x or less. Unlike a lot of players, I'm not going to shove with any Ace or any halfway playable hand once I drop below 10x. I'm still going consider my position, action in front of me, etc. before I act. But when I do find a worthy hand to play, it's going to be for all my chips, maybe even up to 15x depending on the hand and the way the others at the table are playing.
